Katherine Langford as Hannah Baker in a scene from the series 13 Reasons Why on Netflix.Photograph: Beth Dubber/AP

A children’s charity is providing a growing number of counselling sessions for young people concerned about the content of the Netflix drama 13 Reasons Why.

The NSPCC said some children who had received counselling via its Childline service said the series had triggered memories of suicidal thoughts. Others were worried that the programme did not offer advice on how to help someone who was feeling suicidal.

The drama has been criticised by mental health groups for its portrayal of a 17-year-old’s suicide, which they say could encourage young people to take their own lives.

A number of schools have sent letters to parents alerting them to the series, among them St Catherine’s independent girls’ school near Guildford in Surrey. The letter says the series has an 18 rating but that younger girls have watched it and may be encouraging others to do so.

“We don’t want to scaremonger but we do want to flag up what’s out there,” a spokeswoman for the school said. “It was a step we took along with other schools we work with to alert parents to the risks. We’ve had a very positive response from parents.”

Netflix has added an additional warning at the start of the series – on top of warnings already in place for specific episodes – in response to concerns from mental health campaigners.

An NSPCC spokesperson said: “No child should ever feel so helpless that they find themselves at such a crisis point, and we want young people to know they can talk to Childline about anything, anytime on 0800 1111 or childline.org.uk.”

The US-set series is based on a novel of the same name by Jay Asher and is made up of 13 episodes featuring 13 friends of the central character, Hannah Baker, listening to a tape she made for each of them, explaining the difficulties she faced that prompted her to kill herself.

The NSPCC spokesperson said: “Schools must be alive to issues that are affecting their children, and we welcome open communication between teachers and parents.

“Raising concerns, sharing information, and flagging worries can all help adults care for young people and stay abreast of the issues that young people face online. If any adult has a concern about a child they can always call the NSPCC Helpline on 0808 800 5000.”

In the UK, the Samaritans can be contacted on 116 123. In the US, the National Suicide Prevention Hotline is 1-800-273-8255. In Australia, the crisis support service Lifeline is on 13 11 14. Hotlines in other countries can be found here